Induction of an immune response requires T cell expansion, differentiation, contraction and establishment of T cell memory. T cells must encounter antigen presenting cells (APCs) and communicate via T cell receptor (TCR)/major histocompatibility complex (MHC) interactions on APCs. Once the TCR/MHC interaction is established, other sets of receptor-ligand contacts between the T cell and the APC are required, i.e. co-stimulation via CD154/CD40 and CD28/B7.1-B7.2. The synergy between these contacts is suggested to result, in vivo, in a productive immune response capable of clearing pathogens and tumors, and in some cases capable of inducing autoimmunity.
Another level of control has been identified, namely regulatory T cells (Treg). This specific subset of T cells is generated in the thymus, delivered into the periphery, and is capable of constant and inducible control of T cells responses in vitro and in vivo (Sakaguchi (2000) Cell 101(5):455-8; Shevach (2000) Annu. Rev. Immunol. 18:423-49; Bluestone and Abbas (2003) Nat. Rev. Immunol. 3(3):253-7). Treg are represented by a CD4+CD25+ phenotype and also express high levels of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), OX-40, 4-1BB and the glucocorticoid inducible TNF receptor-associated protein (GITR) (McHugh, et al. (2002) Immunity 16(2):311-23; Shimizu, et al. (2002) Nat. Immun. 3(2):135-42). Elimination of Treg cells by 5 day neonatal thymectomy or antibody depletion using anti-CD25, results in the induction of autoimmune pathology and exacerbation of T cells responses to foreign and self-antigens, including heightened anti-tumor responses (Sakaguchi, et al. (1985) J. Exp. Med. 161(1):72-87; Sakaguchi, et al. (1995) J. Immunol. 155(3):1151-64; Jones, et al. (2002) Cancer Immun. 2:1). In addition, Treg have also been involved in the induction and maintenance of transplantation tolerance (Hara, et al. (2001) J. Immunol. 166(6):3789-3796; Wood and Sakaguchi (2003) Nat. Rev. Immunol. 3:199-210), since depletion of Treg with anti-CD25 monoclonal antibodies results in ablation of transplantation tolerance and rapid graft rejection (Jarvinen, et al. (2003) Transplantation 76:1375-9). Among the receptors expressed by Treg, GITR seems to be an important component since in vitro or in vivo ligation of GITR on the surface of Treg with an agonistic monoclonal antibody results in rapid termination of Treg activity (McHugh, et al. (2002) supra; Shimizu, et al. (2002) supra), also resulting in autoimmune pathology (Shimizu, et al. (2002) supra) and ablation of transplantation tolerance.
Costimulatory and co-inhibitory ligands and receptors not only provide a “2nd signal” for T cell activation, but also a balanced network of positive and negative signal to maximize immune responses against infection while limiting immunity to self. The best characterized costimulatory ligands are B7.1 and B7.2, which are expressed by professional APCs, and whose receptors are CD28 and CTLA-4 (Greenwald, R. J., Freeman, G. J., and Sharpe, A. H. (2005). Annu Rev Immunol 23, 515-548; Sharpe, A. H., and Freeman, G. J. (2002) Nat Rev Immunol 2, 116-126). CD28 is expressed by naïve and activated T cells and is critical for optimal T cell activation. In contrast, CTLA-4 is induced upon T cell activation and inhibits T cell activation by binding to B7.1/B7.2, thus impairing CD28-mediated costimulation. CTLA-4 also transduces negative signaling through its cytoplasmic ITIM motif (Teft, W. A., Kirchhof, M. G., and Madrenas, J. (2006). Annu Rev Immunol 24, 65-97; Teft, W. A., Kirchhof, M. G., and Madrenas, J. (2006). Annu Rev Immunol 24, 65-97. B7.1/B7.2 KO mice are impaired in adaptive immune response Borriello, F., Sethna, M. P., Boyd, S. D., Schweitzer, A. N., Tivol, E. A., Jacoby, D., Strom, T. B., Simpson, E. M., Freeman, G. J., and Sharpe, A. H. (1997)) Immunity 6, 303-313; Freeman, G. J., Borriello, F., Hodes, R. J., Reiser, H., Hathcock, K. S., Laszlo, G., McKnight, A. J., Kim, J., Du, L., Lombard, D. B., and et al. (1993). Science 262, 907-909), whereas CTLA-4 KO mice can not adequately control inflammation and develop systemic autoimmune diseases (Chambers, C. A., Sullivan, T. J., and Allison, J. P. (1997) Immunity 7, 885-895; Tivol, E. A., Borriello, F., Schweitzer, A. N., Lynch, W. P., Bluestone, J. A., and Sharpe, A. H. (1995) Immunity 3, 541-547; Waterhouse, P., Penninger, J. M., Timms, E., Wakeham, A., Shahinian, A., Lee, K. P., Thompson, C B., Griesser, H., and Mak, T. W. (1995). Science 270, 985-988.
The B7 family ligands have expanded to include costimulatory B7-H2 (ICOS Ligand) and B7-H3, as well as co-inhibitory B7-H1 (PD-L1), B7-DC (PD-L2), B7-H4 (B7S1 or B7x), and B7-H6 Brandt, C. S., Baratin, M., Yi, E. C., Kennedy, J., Gao, Z., Fox, B., Haldeman, B., Ostrander, C. D., Kaifu, T., Chabannon, C., et al. (2009) J Exp Med 206, 1495-1503; Greenwald, R. J., Freeman, G. J., and Sharpe, A. H. (2005) Annu Rev Immunol 23, 515-548.
Inducible costimulatory (ICOS) molecule is expressed on activated T cells and binds to B7-H2 Yoshinaga, S. K., Whoriskey, J. S., Khare, S. D., Sarmiento, U., Guo, J., Horan, T., Shih, G., Zhang, M., Coccia, M. A., Kohno, T., et al. (1999). Nature 402, 827-832. ICOS is important for T cell activation, differentiation and function, as well as essential for T-helper-cell-induced B cell activation, Ig class switching, and germinal center (GC) formation Dong, C., Juedes, A. E., Temann, U. A., Shresta, S., Allison, J. P., Ruddle, N. H., and Flavell, R. A. (2001) Nature 409, 97-101; Tafuri, A., Shahinian, A., Bladt, F., Yoshinaga, S. K., Jordana, M., Wakeham, A., Boucher, L. M., Bouchard, D., Chan, V. S., Duncan, G., et al. (2001) Nature 409, 105-109; Yoshinaga, S. K., Whoriskey, J. S., Khare, S. D., Sarmiento, U., Guo, J., Horan, T., Shih, G., Zhang, M., Coccia, M. A., Kohno, T., et al. (1999) Nature 402, 827-832. Programmed Death 1 (PD-1) on the other hand, negatively regulates T cell responses. PD-1 KO mice develop lupus-like autoimmune disease, or autoimmune dilated cardiomyopathy depending upon the genetic background Nishimura, H., Nose, M., Hiai, H., Minato, N., and Honjo, T. (1999) Immunity 11, 141-151. Nishimura, H., Okazaki, T., Tanaka, Y., Nakatani, K., Hama, M., Matsumori, A., Sasayama, S., Mizoguchi, A., Hiai, H., Minato, N., and Honjo, T. (2001) Science 291, 319-322. The autoimmunity most likely results from the loss of signaling by both ligands PD-L1 and PD-L2. Recently, CD80 was identified as a second receptor for PD-L1 that transduces inhibitory signals into T cells Butte, M. J., Keir, M. E., Phamduy, T. B., Sharpe, A. H., and Freeman, G. J. (2007) Immunity 27, 111-122. The receptor for B7-H3 and B7-H4 still remain unknown.
The best characterized costimulatory ligands are B7.1 and B7.2 and they belong to the Ig superfamily which consists of many critical immune regulators, such as the B7 family ligands and receptors. Ig superfamily members are expressed on professional antigen-presenting cells (APCs), and their receptors are CD28 and CTLA-4. CD28 is expressed by naïve and activated T cells and is critical for optimal T-cell activation. In contrast, CTLA-4 is induced following T-cell activation and inhibits T-cell activation by binding to B7.1/B7.2, impairing CD28-mediated costimulation. B7.1 and B7.2 knockout (KO) mice are impaired in adaptive immune response, whereas CTLA-4 KO mice cannot adequately control inflammation and develop systemic autoimmune diseases. Over time the B7 family ligands have expanded to include costimulatory ligands such as B7-H2 (ICOS Ligand) and B7-H3, and coinhibitory ligands such as B7-H1 (PD-L1), B7-DC (PD-L2), B7-H4 (B7S1 or B7x), and B7-H6. Accordingly, additional CD28 family receptors have been identified. ICOS is expressed on activated T cells and binds to B7-H2. ICOS is a positive co-regulator, important for T-cell activation, differentiation and function. On the other hand, programmed death 1 (PD-1) negatively regulates T cell responses. PD-1 KO mice developed lupus-like autoimmune disease, or t dilated cardiomyopathy. In contrast to VISTA (the immunosupressive molecule which is the focus of present invention), the two inhibitory B7 family ligands, PD-L1 and PD-L2, have distinct expression patterns. PD-L2 is inducibly expressed on DCs and macrophages, whereas PD-L1 is broadly expressed on both hematopoietic cells and nonhematopoietic cell types. Consistent with the immune-suppressive role of PD-1 receptor, studies using PD-L1−/− and PD-L2−/− mice have shown that both ligands have overlapping roles in inhibiting T-cell proliferation and cytokine production. PD-L1 deficiency enhances disease progression in both the non-obese diabetic (NOD) model of autoimmune diabetes and the murine model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE). FD-L1−/− T cells produce elevated levels of the proinflammatory cytokines in both disease models. In addition, studies in NOD mice have demonstrated that the tissue expression of PD-L1 (i.e. within pancreas) uniquely contributes to its capacity of regionally controlling inflammation. PD-L1 is also highly expressed on placental syncytiotrophoblasts, which critically control the maternal immune responses to allogeneic fetus.
Given the powerful impact of this family of molecules on regulating immunity, substantial efforts in murine models of cancer have shown that protective anti-tumor immunity can be induced by targeting this family of molecules. Studies involving anti-CTLA-4 have documented enhanced therapeutic benefit in murine models and clinical trials of melanoma. Mice vaccinated with B16-GM-CSF (Gvax) promote the rejection of B16 melanomas when combined with antibody blockade of CTLA-4. Antibodies to PD-1 as well as PD-L1 also document enhanced anti-tumor immunity and host survival in a wide range of murine tumor models. Finally, although CTLA-4 and PD-1 belong to the same family of co-inhibitory molecules, evidence suggests they use distinct nonredundant mechanisms to inhibit T-cell activation, and there is synergy in the ability of anti-CTLA-4 and anti-PD-1/L1 to enhance host survival in murine melanoma when used in combination.
Based on the foregoing, the elucidation of another novel B7 type family member and ligands and modulators thereof would be useful given the important role of these family members in regulating immunity, especially T cell immunity.